Replaced "using std::swap" by "using boost::swap" within optional::swap member function, hoping to fix GCC test failures, as mentioned at http://article.gmane.org/gmane.comp.lib.boost.devel/174350 "Re: [optional] problems with swap()"

[SVN r44826]

libs/optional

@@ -1 +1 @@
-Subproject commit a4572497bef424f4db0ceb4fe75661ce0218611a
+Subproject commit 74674531c880f34773ecfc9c89e7198b84e6fd72